Anti-buildup liner

Details Anti-buildup liner Anti-buildup liner

2000-03-17

2003-03-25

Kastler, Scott (Department: 1742)

Metallurgical apparatus

Linings

C222S606000

Reexamination Certificate

active

06537486

ABSTRACT:

FIELD OF THE INVENTION
This invention relates generally to refractory articles and, more particularly, to a refractory shape for transferring molten metal in a continuous casting operation.
DESCRIPTION OF THE PRIOR ART
Refractory shapes are commonly used to control the flow of molten steel in continuous casting operations. Such shapes will often have an inner surface defining a bore through which the molten steel may flow. These shapes may be, for example, nozzles and shrouds, and often are made from a first composition comprising at least one refractory oxide and graphite combined in a carbon-bonded matrix. Graphite improves thermal shock resistance of the shape, but oxidation of the graphite can lead to excessive erosion. A typical fist composition comprises alumina and a lessor amount of graphite.
Refractory shapes also function to protect the steel from contact with air and the resultant oxidation. To reduce oxygen content in the steel itself, molten steel is often “killed,” that is purged of oxygen, commonly by the addition of aluminum metal. Aluminum metal reacts with dissolved oxygen or iron oxide to form finely dispersed alumina, some of which floats into the slag above the molten metal and some of which remains as dispersed particles in the molten steel.
The presence of alumina in the molten steel can result in the deposition of alumina along the inner surface of the refractory shape. Alumina-graphite refractories, although commonly used in refractory shapes, are very susceptible to alumina deposition. Deposition leads to constriction, and possibly clogging, of the bore. The bore may be unclogged using an oxygen lance; however, lancing disrupts the casting process, reduces refractory life, and decreases casting efficiency and the quality of the steel produced. A total blockage of the bore by alumina decreases the expected life of the refractory shape and is very costly and time-consuming to steel producers. For example, steel having an initially high dissolved oxygen content can limit a shroud to 2-3 ladles due to heavy alumina buildup in the bore.
Various techniques have been tried to reduce alumina clogging. A common industrial technique is the injection of an inert gas, such as argon, into the refractory shape. The inert gas is thought to form a protective barrier between the molten steel and the carbon-bonded refractory. Gas injection requires large volumes of inert gas, complicated refractory designs, and is not always an effective solution. Inert gas at high pressure may also dissolve into the molten metal causing defects, such as pinholes, in the cast steel.
Instead of, or in combination with, inert gas injection, the inner surface of the refractory shape may comprise a second refractory composition or liner that either sloughs off as alumina deposits on the surface or does not interact with the molten steel to form alumina deposits. Compositions that slough off may contain or form low melting point materials. U.S. Pat. No. 5,046,647 to Kawai et al. describes a liner comprising calcia/silica capable of forming a low melting point compound. Calcia, however, is prone to hydration, which may create a potentially explosive condition during use. U.S. Pat. No. 5,060,831 to Fishler et al. teaches a composition consisting of carbon and a homogeneous fused mixture of calcia and zirconia that can form a low melting point eutectic with alumina. Zirconia is described as stabilizing the calcia against hydration. U.S. Pat. No. 5,244,130 claims a liner comprising calcium zirconate, graphite, and stabilized calcium silicate that can form low melting point materials. Although complexing calcia with zirconia and silica may reduce destructive hydration, the calcia may not be available to prevent alumina clogging.
Several patents have attempted to produce a liner that is resistant to alumina deposition. U.S. Pat. Nos. 4,870,037 to Hoggard et al. and U.S. Pat. No. 4,871,698 to Fishler et al. teach a liner that reduces alumina clogging, where the liner consists essentially of SiAlON and graphite. Unfortunately, SiAlON liners are not economical. U.S. Pat. Nos. 5,370,370 to Benson and U.S. Pat. No. 5,691,061 to Hanse et al. teach an anti-clogging liner that is made essentially carbon-free by the controlled oxidation of a carbon-containing material. The absence of carbon is believed to inhibit alumina deposition, but the process necessary to oxidize the carbon and effect the required compositional changes is not always practical.
Anti-clogging liners have also been made with aluminum nitride (AlN) bonded refractories as exemplified by U.S. Pat. No. 5,286,685 to Schoennahl. AlN is produced in situ by firing under a nitrogen atmosphere a shape containing powdered aluminum metal. This process is both dangerous, due to the presence of a reactive metal powder, expensive, and time consuming.
GB 2,135,918 to Rosenstock et al. teaches a magnesia liner. Magnesia does not promote alumina deposition, but does suffer from poor thermal shock resistance, spalling and erosion. To improve thermal shock resistance, JP 2-12664 to Tabata et al. teaches a liner comprising 50-90 wt. % magnesia and 10-50 wt. % carbon. The liner may also comprise up to 20 wt. % of additional components, including, for example, chromia, calcia, alumina, silica and zirconia. Additional components can negatively affect hydration, alumina deposition and thermal shock resistance.
U.S. Pat. No. 5,885,520 to Hoover attempts to combine the benefits of calcia and magnesia. It teaches a carbon-bonded liner comprising doloma and more than 33 wt. % graphite. Doloma comprises approximately 58 wt. % calcia and 42 wt. % magnesia. Adequate thermal shock resistance is achieved only when the graphite content is more than about 33 wt. %, but high amounts of graphite can make the composition susceptible to oxidation and erosion, both of which can cause break-out of molten steel.
A need persists for an inexpensive, easily fabricated refractory composition that reduces alumina deposition while resisting oxidation and erosion. Such a composition would be especially useful on the inner surface of a refractory shape, such as, for example, a liner in the bore of a refractory nozzle or shroud.
SUMMARY OF THE INVENTION
The present invention describes a refractory shape for transferring molten metal in a continuous casting operation. One object of the invention is to decrease the build-up of alumina in the bore of such a refractory shape. A second object is to improve the erosion-resistance of the bore to molten steel. A third object of the invention is to reduce destructive hydration of calcia-rich grains. A fourth object of the invention is to enhance the thermal shock resistance of a liner within the bore while using a reduced amount of carbon.
One aspect of the invention teaches a carbon-bonded refractory shape formed from an unfired composition comprising a calcia-rich grain, a hydration-resistant grain, 6-28 wt. % carbon and a sufficient amount of binder. The calcia-rich grain will typically be dolomite, but may also be, for example, calcia, calcium zirconate, calcium silicate, calcium titanate and their combinations. Preferably, the calcia-rich grain will contain at least about 45 wt. % calcia. The hydration-resistant grain is less prone to hydrate than calcia and does not promote alumina deposition. Examples include magnesia, zirconia, various nitrides and silicates, and combinations thereof.
Another aspect of the invention describes the calcia-rich grain as coarse and the hydration-resistant grain as sufficiently fine so as to fit within the interstices between coarse calcia-rich grains. The hydration-resistant grain may have a multi-modal size distribution to fit within increasingly small interstices.
One embodiment of the refractory shape is a shroud or nozzle. Alternatively, the shape may be any refractory piece having a bore through which the stream of molten steel flows. Typically, the shape will include a first composition comprising the bulk of the shape and a second composition at least partially lining an inner surface that contacts t

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